1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus and method for controlling an electromagnetic contactor.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued August 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged thereby as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts. Generally, as the armature is accelerated towards the magnetic, it must overcome two spring forces. The first spring force is provided by a kickout spring which is subsequently utilized to disengage the contacts by moving the armature in the opposite direction when the power applied to the coil has been removed. This occurs as the contacts are opened. The other spring force is provided by a contact spring which begins to compress as the bridging contacts abut the fixed contacts, but while the armature continues to move towards the fixed magnet as the air gap is reduced to zero. The force of the contact spring determines the amount of electrical current which can be carried by the closed contacts, and furthermore determines how much contact wear is tolerable as repeated operation of the contactor occurs. It is usually desirous for the contact spring to be as forceful as possible, thus increasing the current-carrying capability of the contactor and increasing the capability to adapt for contact wear. However, since this force must be overcome by the energy provided to the electromagnet during the closing operation, more closing energy will generally be required for relatively stiffer contact springs than for less stiff contact springs. Most electromagnets in contactors are powered by alternating current and, as will be described herein more fully hereinafter, the magnet pull curve for the electromagnetic armature accelerating system is generally fixed in shape according to the magnetic system utilized. In prior art contactors, the amount of energy provided to the electromagnet is more than is necessary to overcome the force of the springs against which the accelerating armature operates. One reason for this is the need to overcome the effect of the relatively stiff contact springs when the contacts engage. In general, however, the excess energy is wasted energy which is undesirable. But, perhaps more importantly, the excess energy is absorbed by the mechanical system as the armature finishes its closing travel stroke. This excessive kinetic energy is usually exemplified by heat, noise, vibration, undesirable contact bounce and shock. It would be desirable, therefore, to find an electromagnetic closing system which provided only the amount of energy necessary to overcome the forces which resist movement of the armature in the closing stroke and no more. Furthermore, it would be desirable to reduce the velocity of the armature to a relatively low value such as zero as it abuts the fixed magnet. The lower the velocity the less the movable contact bounces when it abuts the fixed magnet.